Not applicable.
Not applicable.
The present invention relates to papermaking headboxes in general, and in particular to vanes or headbox trailing elements which are used to divide the flow of stock in a slice. The headbox of a papermaking machine injects a stream of paper forming stock onto a moving forming fabric or between twin forming fabrics in the forming section of the papermaking machine. In the modem papermaking machine the forming fabric or wire may be moving in excess of 6,000 feet per minute. Moreover, the forming fabric, sometimes referred to as a forming wire, may be more than 400 inches wide. Thus the jet of water must exit the headbox as a sheet of water containing pulp moving approximately 100 feet per second and more than 30 feet wide. Great uniformity across the sheet of paper, and along the direction in which the paper is formed is required. The headbox functions to provide a uniform flow of stock, and to accelerate the stock through a slice to form a jet which is emitted from the lip of the slice. Paper fibers suspended in water have a tendency to come together in clumps, with highly undesirable effects on the uniformity of the paper web formed. Various hydraulic structures located within the headbox are designed to create turbulence to prevent clumping or flock creation. Vanes within the headbox, i.e., flexible plastic sheets extending from the headbox injection wall to near the slice outlet, provide small-scale turbulence which prevents the formation of flocks.
As a result of wear or accident, the plastic vanes require periodic replacement, and thus recurring cost is a significant concern. The vane plastic sheet may be formed of polycarbonate, Hylar(copyright) PVDF fluoropolymers, graphite composite, or glass fiber composite, and is attached to a bracket which pivotally mounts the plastic sheet to the injection face of the headbox. The bracket has cylindrical portions that fit within a dovetail groove machined into the injection face of the headbox.
Forming a vane with a bead increases the complexity and cost of fabricating the vane. An added complication is presented in headboxes where the injection wall is not perpendicular to the jet formed at the outlet of the slice. In such a case it is necessary to form an angle at the transition between the retaining bead and the vane which lies generally parallel to the jet. One approach to mounting vanes within a headbox has employed a hinge bracket which is hingedly mounted within the groove in the injection face and connects to an essentially planar vane. The prior art includes a bronze bracket which functions satisfactorily, but has a high initial cost. Thermal expansion is also a concern because the temperature of the stock is at times varied, and bronze has a relatively fixed coefficient of thermal expansion. The bronze bracket must be machined from expensive bronze stock resulting in considerable cost. The cost of the bronze bracket necessitates that the bronze brackets be reused. Considerable expense can be involved in returning to the factory bronze brackets which may be more than thirty feet long.
What is needed is a vane hinge bracket which is low-cost and which has tailorable thermal expansion properties.
The vane hinge bracket of this invention is composed of graphite epoxy, and has a cylindrical portion which hingedly fits within a dovetail groove which extends in the cross machine direction across the injection face of a papermaking headbox. The vane hinge bracket has downstream portions forming a vane receiving slot, with a plastic sheet forming the vane positioned within the receiving slot. The plastic vane is held within the receiving slot by a plurality of metal screws which extend perpendicularly through holes formed in the plastic sheet and perpendicular to the slot formed on the downstream side of the vane hinge bracket. The metal screws extend through holes formed in the portions of the vane hinge bracket which define the slot. A web of material extends between the downstream portions and the cylindrical portions, the web may be curved so as to hold a vane an angle with respect to the injection face, or may be straight.
The vane hinge bracket is laid up of carbon fibers oriented so that the coefficient of thermal expansion matches the steel from which the headbox is manufactured. A two-part metal mold is used to define the underside and the bead portion of the bracket, a rubber gasket of Shore 70 material forms the center of the bead portion about which prepregs of graphite cloth are wrapped. A slot is formed by a polytetrafluoroethylene insert which is removed once the vane hinge bracket is cured. The two-part mold containing the prepregs, the rubber cord, and the Polytetrafluoroethylene insert are placed under a vacuum bag and the two-part mold is heated. Heating the mold causes the rubber cord to expand, pressing the overlying graphite prepregs against the sides of the mold. By proper orientation of the fibers, the thermal expansion can be matched to the steel from which the headbox is formed with a linear coefficient of thermal expansion of about 6.5 ppm per degree Fahrenheit.
It is a feature of the present invention to provide a hinge bracket for mounting a vane in a headbox which is of lower cost.
It is a further feature of the present invention to provide a hinge bracket with a coefficient of thermal expansion which can be tailored.
It is yet another feature of the present invention to provide a hinge bracket for mounting a vane in a headbox with a simplified attachment means.
It is another feature of the present invention to provide a method of manufacturing a hinge bracket for mounting a vane in to a headbox.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.